As a result of increasing demands for purification of combustion engine exhaust, combined with requirements for compact installation in many applications, for instance that of automotive exhaust systems, silencers are nowadays often designed to contain built-in purification equipment, such as particle filters and catalysers based on ceramic monoliths. Also, silencers are sometimes required to contain heat exchangers for the extraction of exhaust heat, for cabin heating or cooling, by means of a heat-driven chiller, such as an absorption chiller. When exhaust gas flows through such ceramic monoliths and heat exchangers, the flow is typically being divided into many small, parallel sub-flows. Accordingly, these elements can be designated as porous bodies.
Reactive silencers basically function as acoustical low-pass filters, i.e. they provide noise reduction at frequencies above a lower cut-off frequency f″ below which there is no or little attenuation. In addition, the transition from no to full attenuation is often gradual, characterised by a second cut-off frequency f′, which is somewhat higher than f″. Such a second cut-off frequency typically occurs in the case of a silencer with two acoustical chambers being connected by an internal pipe. From acoustical theory it is known that f′ and f″ more or less coincide with natural oscillation frequencies, known as Helmholtz frequencies.
As discussed below in connection with FIG. 1, the natural (and cut-off) frequency can be lowered if connecting pipe length L′ is made longer. This would result in improved low-frequency noise reduction, as discussed below in connection with FIG. 1.
However, with silencers of simple geometry, as indicated by the schematic of the figure, there is a limit to the possible length L′, being ultimately the length of the casing, i. e. the sum of the lengths of the two chambers. In practice, since flow in and out of chambers has to be provided in a reasonable way, the limit length actually is lower, typically in the order of half the casing length or slightly more.
International Patent Applications Publication Nos. WO 98/14693 and WO 99/50539 provide solutions to this problem. A main idea disclosed in these patent applications is to use a curved, internal passage instead of a straight passage. It is shown how helical passages, extending inside a silencer close to the casing and winding e.g. 360 degrees, can result in a substantial increase in effective passage length, which is measured along the curved path from inlet (connected to a first chamber) and to passage outlet (connected to a second chamber).
These cited publications show that the principle of a curved passage, used with the purpose of enhancing low-frequency acoustical performance of two- or more chamber reactive silencers, can be applied both to classical silencers and to silencers containing monoliths. In the latter case, monoliths are shown to be connected in series with such curved passages and contained inside an acoustical chamber, having a diameter being only slightly less than that of the casing and being fixed to the casing, directly or via a heat-resistant, flexible layer. Such series connection of curved passages and monoliths, though, demands that the monoliths do not occupy too big a part of the total volume inside the silencer casing, assuming that a reasonably unrestricted flow in and out of chambers must be accommodated for.
In silencers containing monoliths, passages connecting acoustical chambers may be designed as annular passages surrounding such monoliths, instead of pipes. For instance, U.S. Pat. No. 5,426,269 teaches that such a passage can be used for leading gases along the outer cylinder of a catalytic monolith, in counterflow to flow through the monolith, in a combined silencer/catalyser having inlet and outlet pipes essentially at the same end of a cylindrical casing.
International Patent Application Publication No. WO 97/43528 further demonstrates how an annular passage surrounding one or more monoliths disposed inside a silencer and being penetrated by a central pipe, can be combined with accommodation of a rather long passage connecting two chambers. Here, the main purpose is to achieve a low cut-off frequency, as with curved, internal passages. Inlet and outlet pipes are connected to opposite ends of the casing. One of the embodiments shows how two monoliths, being for instance a particulate filter and a NOx-reducing catalyser, can be accommodated inside an extremely compact combined unit according to this invention.
This latter concept is especially attractive in cases where there is space for using a rather longish casing, because in such apparatuses the annular passage can attain a substantial length, constituting a rather low cut-off frequency associated with this passage. But in short silencers, the cut-off frequency goes up.